If you search the forum there are supposed to be newer DVRs that have double the space available and a firmware update is supposed to enable the E-SATA ports on the 7xxx series DVRs.
You should be able to get about 20 hours of HD or 80 hours of SD (or some combination of the 2) with the current DVR (160 GB drive). Keep in mind that the firmware is notorious for reporting the wrong space used. If it looks like the reporting is much more than about 5% per hour of HD then try rebooting the DVR (either from the menu or by pulling the power for 15 seconds).
Remember, search is always your friend.
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Thanks. I just got FIOS installed. One DVR says it's recorded 1 hr of HD and is 98% free. The other says it's recorded 7 hrs and is onlly 47% free. Knowing these numbers aren't necessarily correct makes me feel better. 🙂